Currently, there are significant technical challenges in providing hermetic coatings or other protective layers on polymer materials, plastic substrates or sensitive inorganic materials. Some commercial applications are protective coatings for thin film photovoltaic panels, especially those having organic photovoltaic converting materials, or inorganic PV materials such as Copper Indium Gallium di-Selenide (CIGS) and others. Another major and challenging application is to form protective layers having very few defects or “pinholes” to cover active matrix OLED screens or lighting panels. Yet another application is to make anti-reflection or protective coatings on substrates.
In order for vacuum-based plasma coating process to be economically competitive the total cost for the deposition process must always be low enough that the products made using them are competitive. Such coating processes may be vacuum-based or atmospheric pressure processes using a liquid form to spread across the substrate. While liquid-based application may be cheaper to apply it often requires extensive drying/curing operations and usually cannot produce very thin coatings that are sometimes needed. In cases where coatings must be very durable or have special chemical bonding or optical properties they sometimes can only be made with vacuum-based plasma deposition processes. For various such applications there are widely differing cost requirements which may range from about $1/square meter for very thin hard coatings or amorphous silicon passivation coatings for photovoltaic panels, to more than $100/square meter for multi-layer dielectrics, or for thicker metal oxide or metal nitride coatings. In some cases, the manufactured product requires very large substrates to give the needed product performance or economy of scale. Good examples of such are thin film photovoltaic devices, films for windows or display screens. For a coating technology to be cost effective in such applications it must also be able to be scaled up while maintaining needed uniformity of coating properties for substrates two meters square in size, or larger.
One such type of critical application is for hermetic coatings for Organic Light Emitting Diode (OLED) materials for display screens or lighting. Such materials must be protected by very tight hermetic barriers for both oxygen and water vapor. Manufacturing of OLED or organic photovoltaics, is typically done on large substrates or continuous webs. Hermetic barriers, which must keep atmospheric gases out of a covered layer or substrate material, must be done at temperatures that do not damage the light emitting property of the polymer. Second, and equally important, is that, in the coating, there be extremely low defects that permit moisture or gases to come through the coating to damage the sensitive material underneath. Thirdly, the coating should be uniform in thickness and composition so that it has the same required properties over the entire area of the substrate and devices that will be made from it.
A low temperature coating process is required that also has extremely low defect density—much less than ten per square meter of substrate area—so that minimal areas are affected by the resultant leaks. For OLED devices the maximum tolerable temperature for deposition of needed hermetic barrier layers or overlying metal oxide layers, either conducting or semi-conducting, is between about 70° C. and about 90° C. Typically, barrier layers may include dielectrics such as silicon nitride or silicon oxynitride or other silicon-based materials, and in some cases, carbon based materials. Conducting metal oxides include zinc oxide, tin oxide, indium-tin oxide and some others. Semiconducting metallic oxides are more complex typically using oxides of three metals—such as indium, gallium and zinc or indium, tin and zinc.
Other applications involve coating of plastics or polymer coated substrates. For some less temperature-tolerant polymers, such as PMMA, PVC, Nylon or PET, coating processes must be done with maximum tolerable temperature between about 75° C. and about 100° C. Among the common and useful coatings for such plastics are dielectric coatings for scratch resistance and optical coatings for anti-reflection as well as selective transmission of different bands of visible and infrared light. Coatings on some other more stable plastics such as PEN and epoxies must usually be done at temperatures less than 125° C. This is also a general upper temperature limit for some other polymers such as polystyrene used for organic photovoltaics and some semiconductor packaging applications. Acceptable processing temperatures are typically over 300° C. for glass, or up to about 300° C. for some few unusual plastic materials such as PFA or PEEK. Temperatures up to a limit of about 300° C. may be acceptable for depositing metal oxides on various metal substrates or webs. Currently, the leading process involves applying alternate layers of organic polymer and sputtered aluminum oxide. This process works well for small display but is not economical for larger screens due in large part to the limits defects introduced by the sputtering process. State-of-the-art defect density with sputtering is between about ten and fifty defects per m2. This areal density of defects is not adequate even for screens as small as those for “pad” devices, let alone notebook computers where yields would be less than one good screen for per five manufactured.
The material needing protection may be of many types, including, but not limited to, organic materials or plastics for light emitting diodes, photovoltaic or solar concentrators, or inorganic materials used for electronics or photovoltaics. Substrate type may be silicon or other inorganic wafers, individual plates of glass or plastic, or be a long roll of material that is best processed continuously. Further, coatings applied using such technologies have general characteristics, strengths and limitations which make them more or less specific to each of the different types of applications.
Reactors for plasma enhanced coating of substrates include both cluster and in-line architectures. Deposition technologies including parallel plate PECVD, microwave plasma and sputter coating have been used for both conducting and dielectric thin films. Sputtering has been the most common type of deposition technology used for making very thin coatings at low temperature but this technology often has problems with cleanliness and can also cause excessive heating of the substrate due to the inability to remove heat from the substrate at the low reactor gas pressures required for sputtering processing. Sputter coaters have been used for many years for large and small substrates. Among those available have been in-line systems by manufacturers from Airco/Temescal to more recent systems from Veeco, FHR/Centrotherm, or Vitex Systems. PECVD is an alternative but has not been able to make good quality films at substrate temperatures less than about 200° C. Such systems include such as the Applied Materials cluster reactor for deposition of silicon and silicon nitride thin films in LCD screen manufacture, or in-line systems such the Roth & Rau system for coating solar cell wafers with silicon, or dielectrics such as silicon oxide. Scaling such reactors to process ever larger substrates has made it increasingly difficult to maintain the desired film properties and uniformity of thickness of the coating across the entire substrate.
Dielectric coatings at temperatures below about 200° C. are generally deposited by sputter processes. Sputtering can be used for coatings at even at lower substrate temperature, below 100° C., but the deposited films often exhibit a columnar structure. The columnar structure is not desired for barrier films since the defective region surrounding each column extends across the thickness of the film allowing for high rates of diffusion/penetration by gas or liquid. Accelerating ions towards the substrate by applying bias during the sputtering process adds energy to the atoms on the surface of the depositing film. The added energy by impinging ions allow the atoms on the surface of the depositing film to move around, providing for a more isotropic film structure and higher film density. However, the low process chamber pressure during sputtering makes it difficult to dissipate the heat added to the substrate by impinging ions. The methods to control substrate temperature during sputtering developed for integrated circuit processing, such as electrostatic chucks and backside He flow, are not practical or economical for substrates that are large, made from dielectric materials, or continuously moving. RF plasma-based PECVD on the other hand tends to make denser films with more controllable stress and amorphous structure but typical implementations require substrate temperatures above about 180° C. The elevated substrate temperature is required to complete the chemical reactions involved in the deposition process to reduce incorporation of unwanted species such as hydrogen, water, and un-reacted precursor ligands. Increasing the RF frequency above the typical 13.56 MHz may improve the efficiency of breaking down the precursors and completing the chemical reaction. For example, microwave deposition systems typically produces coatings at a higher rate and more efficiently from the gas feedstock, but the coatings tend to be less dense, more tensile in film stress and may not adhere well to the underlying material.
In RF-plasma-based PECVD gas phase particles typically become negatively charged and suspended away from the substrate in high field regions at the plasma/sheath boundaries. In addition the internal surface of a plasma based process chamber can also be conveniently cleaned by running a plasma based chamber clean recipe. By injecting process gasses that can be activated to etch away deposits inside the chamber that can flake off and become particles or defects on the processed substrates. The intervals between chamber cleans are determined as a balance of maximizing productivity against the chance that accumulating of deposits inside the process chamber creating particles on the substrate. The plasma distribution during the processing step can be made to match the distribution during the cleaning process ensuring that cleaning is efficiently performed by focusing on the areas that need cleaning the most. The excellent particle performance of plasma based processes is demonstrated in semiconductor manufacturing of nanometer scale devices where less than about 5 particles larger than 50 nm size on wafers of 300 mm diameter is a normal operating result. Sputtering processes and chambers typically have particle densities on substrates an order of magnitude greater than plasma based processes. The reason is that in sputtering systems there is no inherent tendency for particles to be captured before ending up on the substrates and in-situ cleaning methods are not as easily incorporated in to sputtering systems. Chamber cleaning for sputtering systems is typically based on switching out internal shield surfaces inserted in the process for the purpose of absorbing deposition fluxes that do not end up the substrate. The films ending up on these shield surfaces may be come stressed and prone to flake off, causing large particle “dumps” on to the substrates. Cleaning of sputtering systems also takes longer because each time the process chamber must be vented, opened, parts replaced, maybe some manual wiping, closed back up, and pump/purged to get back to production.
The prior art does not provide deposition systems that can deposit dense quality encapsulation films at high-rate and low-cost with low defect density while at the same time maintaining temperatures below 100° C. There is, therefore, a need for improved processing technology to meet these needs and at the same time be compatible with high-volume production.